This invention relates to a heat exchanger produced by using aluminum or its alloys in part or whole of the component parts of heat exchanger which has winding heat exchange tubes fixed by a plurality of spaced plate-fins and is particularly excellent in corrosion resistance and heat exchange properties.
Aluminum and its alloys are widely used in the field of heat exchangers because they are light in weight, excellent in workability, corrosion resistance and heat conductivity, and less expensive than copper materials. Heat exchangers made of aluminum or its alloys are usually used in air-conditioners for cooling and heating.
When an air-conditioner is operated for cooling, the moisture in the air is condensed to water and adheres to the surfaces of aluminum fins of the heat exchanger by dehumidification. The water drops adhered to the surfaces of fins are present in the form of semicircle or bridging state between fins due to poor hydrophilic nature of the fins, which results in preventing smooth flow of air and increasing resistance to flow.
On the other hand, aluminum and its alloys are excellent in corrosion resistance. But when condensed water remains on the aluminum fins for a long period of time, the formation of an oxygen concentration cell or gradual adsorption and condensation or contaminants in the atmosphere accelerates the hydration reaction and corrosion reaction. Corrosion product is deposited on the surfaces of fins, which not only affects heat exchange properties badly but also undesirably releases a white fine powder delaminated from the fins together with warm air from the exhaust grill with the air-conditioner is operated for heating in winter.
Therefore excellent corrosion resistance is required for the aluminum fins constituting the heat exchanger. Heretofore, there have been employed various methods of corrosion proofing treatments by forming organic resin protective films or chemical protective films (the boemite method, the chromate method, and the like). But such corrosion proofing treatments give various defects, such as hydrophilic properties (wettability) of the aluminum surface being reduced, condensed water being easily adhered to fin surfaces when the air-conditioner is operated for cooling, and the air flow resistance of the heat exchanger being remarkably increased. In view of these defects the noise increases and the performance is lowered.
In such a case, if the aluminum fins are coated with a paint and the condensed water is completely absorbed on the coated film or the coated film is wetted uniformly by the condensed water, such defects as mentioned above may be overcome. Alternatively, the condensed water may completely be repelled by the coated film so as not to make the water to adhere to the coated film.
As the prior art, there are known Japanese Patent Appln Kokai (Laid-Open) Nos. 57264/79 (a heat exchanger), 1450/79 (a cooler) and 14450/78 (a heat exchanger having good hydro-extraction properties). According to Kokai No. 57264/79, the surface of heat exchanger produced by using aluminum or its alloys is treated with an aqueous solution containing silicate compounds such as water-soluble or water-dispersible silicates, and subsequently treated with an alkaline aqueous solution containing one or more alkaline earth metal compounds such as hydroxides, oxides, chlorides, acetates, nitrates and the like of alkaline earth metals to form a chemical coating on the surface of aluminum or its alloys so as to improve corrosion resistance of the aluminum fins and at the same time to increase hydrophilic properties of the fin surface, so that the heat exchanger having improved durability and performance is obtained. According to Kokai No. 1450/79, aluminum fins are dipped in an alkaline treating solution containing one or more organic acid such as tannic acid, and the like at 15.degree.-45.degree. C. for 30 to 90 seconds to form crystalline coating so as to prevent change in quality due to air oxidation and to make the surface tension small so as not to reduce air flow amount when air is flowed between the fins. According to Kokai No. 14450/78, the aluminum fin surface is treated with boemite (.gamma.-Al.sub.2 O.sub.3.H.sub.2 O or AlOOH) or calcium aluminate, or coated with polymer compounds as a paint such acrylic resins, polyurethane resins, etc., followed by treatment with a surface active agent to provide hydrophilic properties so as to improve hydro-extraction of the aluminum surface and heat exchange efficiency.
But it is remarkably difficult to obtain a coating film which can completely repel condensed water (the contact angle of water drop being 90 degrees or larger). On the other hand, a coating film which can absorb condensed water, i.e. so-called hydrophilic coating film is usually insufficient in surface hardness and since the coating film contains a large amount of water inside of the film under humid air condition, surface hardness of the coating film is remarkably lowered due to swelling of the resin in the coating film and wear resistance is also lowered; this becomes one factor for lowering corrosion resistance.
As mentioned above, since there is an inconsistent relationship between the hydrophilic properties and the surface hardness, and corrosion resistance, it has been a great problem to find a balanced point or compromising point.